"This is the beginning of her preparation for Wimbledon," Patrick Mouratoglou said. "It's the start of the fightback. She will be furious about this match, furious about what she's done. She will be even more determined to go out and win Wimbledon and everything else that follows."

Williams' loss to Virginie Razzano in the first round here two years ago - after which the American went on to win Wimbledon, the Olympics, the US Open and the year-ending WTA Championships - was arguably the most shocking of her career, but the defeat by Muguruza was her heaviest in her 288 Grand Slam matches.

The world No 1 played abysmally. She pulled out of the recent Madrid tournament with a leg injury and barely looked to be moving in the latter stages of this match, but the player and her coach insisted that she had not had a physical problem.

"It was one of those days," Williams said. "You can't be on every day and, gosh, I hate to be off during a Grand Slam, but it happens. It's not the end of the world."

Mouratoglou said he had no explanation for Williams' performance but added: "She was the overwhelming favourite, the title holder, she has a 4,000-point lead at the top of the world rankings. The pressure is enormous. In that situation it's often the early rounds that are the most dangerous. We've seen in this tournament that there are a lot of players who have succumbed to pressure in the early rounds."

With Li Na, the world No 2, having fallen in the first round, this is the first time at any Grand Slam tournament in the Open era that the top two women's seeds have lost before the third round. Williams' sister, Venus, also went out, losing to Slovakia's Anna Karolina Schmiedlova.

Muguruza, a fine athlete with a big game, was born in Venezuela. The 20-year-old's father is Spanish and her mother Venezuelan and she has yet to decide which country to represent in the Fed Cup. She turned professional three years ago and this is only her sixth Grand Slam tournament.

Asked what Williams, her childhood idol, had said to her at the net at the end of the match, Muguruza said: "She said that if I continue playing like this, I can win the tournament. I said: 'I will try, I will try.'"

Novak Djokovic and Roger Federer, the top seeds in the bottom half of the men's draw, both reached the third round with straight-sets victories. Federer, who meets Dmitry Tursunov, beat Diego Schwartzman, while Djokovic overcame Jérémy Chardy to earn a meeting with Marin Cilic.

Andy Murray plays his second-round match today against Marinko Matosevic, a 28-year-old Australian who recorded his first Grand Slam victory in 13 attempts when he beat Dustin Brown in the first round. Matosevic said the win felt like "a huge gorilla off my shoulders" but added: "I always knew I was a better player than my Grand Slam results."

Murray, who beat Matosevic at Queen's Club last summer in their only previous meeting, gets on well with the world No 66, who has been nicknamed "Mad Dog". Matosevic, who drew a laugh from Rafael Nadal when he deliberately kicked over the Spaniard's water bottles during their match in Monte Carlo last year, does not like the nickname. "Some idiot put it on Wikipedia," he said. "Mad Dog, what is that? Like a dog with rabies?"